The present invention relates to an electronic switching device which employs an active layer of an ion impermeable glass.
The term glass within the context of this description, includes those materials which typically exhibit only short-term ordering. It is intended to exclude the true crystalline substances which are the semiconductor materials commonly used in active electronic devices and the true amorphous materials which have no appreciable ordering. Glasses are typically quenched liquids having a viscosity in excess of about 10.sup.8 poise at ambient temperature. They are generally characterized by: (1) the existence of a single phase; (2) gradual softening and subsequent melting with increasing temperature, rather than sharp melting characteristics; (3) conchoidal fracture; and (4) the absence of crystalline X-ray diffraction peaks.
Because of the ease with which they can be fabricated, glass switching and memory devices offer considerable promise for a wide variety of applications in the electronic devices art. A typical glass switching device, such as that described by J. F. Dewald et al., in U.S. Pat. No. 3,241,009 issued Mar. 15, 1966, comprises a layer of a special composition "semiconducting" glass, such as As-Te-I, disposed between a pair of electrodes. In operation this device exhibits switching and memory capability. When an increasing voltage is applied across the layer, a high resistance is encountered until the voltage reaches a critical value V.sub.c. Any additional increase in the voltage switches the glass into a high conducting state. The switching can occur in less than a microsecond and is reversible. The device remains in a given state, even under zero bias, and thus displays memory.
One of the factors which has thus far prevented full realization of the potential of these devices is insufficient long-term stability. (See, for example, J. D. Mackenzie, Looking Through Glasses for New Active Components, 39 Electronics 135, Sept. 19, 1966.) The current-voltage characteristics of the device tend to undergo irreversible changes with prolonged use. It is applicant's belief that these irreversible changes are due, at least in part, to the migration of impurity ions within the glass. (Similar irreversible changes in the characteristics of SiO.sub.2 dielectric layers have already been attributed to the migration of sodium ions.)
Another factor which has limited the usefulness of these glass switching devices is that they are not well-suited for incorporation into integrated circuits. These devices have been typically conceived, fabricated, and utilized as discrete element circuit components. Moreover, they are typically ill-suited--both dimensionally and compositionally--for easy integration into circuits which also contain junction or field effect devices. For example, the planar technique for fabricating integrated circuits generally requires thin masking layers (typically having a thickness of 2 microns or less for high-component density integrated circuits) for facilitating the fabrication of diffused junction devices. In addition, it requires high quality dielectric layers (typically having a thickness of a micron or less) for use in capacitors and surface effect devices, and ion impermeable layers for passivating the underlying crystalline semiconductor substrate. The glass layers used in typical prior art glass switching devices, however, are not well-suited to perform any of these functions. They typically have a minimum thickness in excess of several microns thereby rendering them useless as masking layers; they typically have resistivities between 10.sup.2 and 10.sup.10 ohm-cm thereby reducing their value as dielectrics; and they are not sufficiently ion impermeable at elevated temperatures to provide high quality passivating layers. In addition, many of these glasses do not have coefficients of thermal expansion and contraction which are compatible with typical crystalline semiconductor substrates. Moreover, these glasses typically lack sufficient compositional stability at elevated temperatures to permit the use of conventional fabrication techniques. (See, for example, R. G. Neale et al., Nonvolatile and Reprogramable, the Read Mostly Memory is Here, in the Sept. 28, 1970 issue of Electronics, particularly at page 58.)